Production of aromatic polyesters of improved color

ABSTRACT

A polyethylene terephthalate composition comprising highly polymeric polyethylene terephthalate and in an amount sufficient to prevent color degradation of said polyethylene terephthalate (1) a trihalide or tri (pseudohalide) of antimony, and (2) an organic oxo compound of phosphorus.

This application is a continuation of copending application Ser. No. 877,503, filed Nov. 17, 1969 now U.S. Pat. No. 3,830,775.

This invention relates to a method of improving the colour of highly polymeric polyesters which have been obtained from polycondensations involving the use of antimony-containing polycondensation catalysts.

Highly polymeric polyesters of ethylene glycol and aromatic dicarboxylic acids, by which we mean dicarboxylic acids wherein each--COOH group is attached to a carbon atom which forms part of an aromatic carbocyclic ring, are known to be useful thermoplastic materials. Two which may be mentioned in particular are those from terephthalic acid and 1,2-di(p-carboxyphenoxy)ethane.

Highly polymeric poly(ethylene terephthalate), for example, is a thermoplastic having a very desirable combination of physical and chemical properties and is produced in large tonnages for conversion to filament, fibre and film. More recently, compositions based on it have been announced for general thermoplastic moulding, eg. injection-moulding, applications.

For many applications, and particularly for use in fibre or filament form in textiles, for use in thermo-plastic moulding applications or for use in films, it is highly desirable that these polyesters be as free from discolouration as possible.

Many processes have been proposed for the production of these polyesters and the processes have involved a variety of staring materials. In general, however, all the processes proceed via the formation of a bis ethylene glycol ester of the aromatic dicarboxylic acid and the polycondensation of this intermediate to highly polymeric polyester with loss of ethylene glycol. The bis glycol ester may be formed for example, by reaction of the aromatic dicarboxylic acid or of an ester-forming derivative thereof, e.g. a dialkyl ester, with ethylene glycol, or by the reaction of the aromatic dicarboxylic acid with ethylene oxide or with ethylene carbonate.

Both the process of forming the bis glycol ester and the polycondensation thereof to highly polymeric polyester may be facilitated by the action of catalysts, and in accordance with one preferred and widely used method of effecting the polycondensation, catalysis is provided by compounds of trivalent antimony. The use of these compounds, however, has tended to produce polymer which is discoloured by a greyish tinge which is thought to be due to residual finely divided antimony metal in the polymer. This grey discolouration is not to be confused with the yellowing which has also been observed in these highly polymeric polyesters, and which is generally believed to be due to thermal degradation of the polymer promoted by residues of the catalyst used in the formation of the bis glycol ester. Furthermore, the remedies proposed in the past for inhibiting the yellowing do not seem to provide an answer to the greying referred to. For example, the phosphorus-based additives popularly used for reducing the yellowing, e.g. phosphorus acid and organic phosphites, appear if anything to promote the grey discolouration, especially when used in conjunction with antimony trioxide.

We have now found that this tendency to a grey discolouration associated with the use of antimony compounds may be reduced by using as the polycondensation catalyst an antimony trihalide or tri(pseudohalide) and an organic oxo compound of phosphorus.

So as not to modify, dilute or lose the highly desirable properties of the polyesters of ethylene glycol and aromatic dicarboxylic acids it is usually preferred that the polycondensable material consists substantially entirely of one or more bis ethylene glycol esters of aromatic dicarboxylic acids. However, the presence of a small concentration of other polycondensable material may be tolerated, if desired, for example to improve dyeability. For example, up to about 5 mole % of the ethylene glycol moiety in this bis glycol ester or esters may be replaced by at least one other polycondensable dihydroxy compound and/or up to about 5 mole % of the aromatic dicarboxylic acid moiety may be replaced by at least one another dicarboxylic acid. Up to about 5 mole % of the polycondensable mixture may also consist of other mono- or polyfunctional material, if desired; e.g. alcohols and/or their esters with the dicarboxylic acids, amines and/or diamines and/or their amides with the dicarboxylic acids, aminoalcohols and/or their condensation products with the dicarboxylic acids, and/or amino acids, hydroxy acids, lactams and/or lactones and/or their condensation products with the dicarboxylic acids and/or glycols. However, it is usually preferred that at least 85 mole %, and preferably at least 95 mole %, of the polycondensable mixture consists of bis ethylene glycol ester of aromatic dicarboxylic acid.

Thus, according to the present invention we provide a process for the production of highly polymeric polymer by the polycondensation of polycondensable material at least 85 mole % of which consists of at least one bis ethylene glycol ester of an aromatic dicarboxylic acid in which said polycondensation is effected in the presence as catalyst of

(i) a trihalide or tri(pseudohalide) of antimony, and

(ii) an organic oxo compound of phosphorus.

While the process of our invention is applicable in general to the production of polyesters of ethylene glycol and any aromatic dicarboxylic acid, it is especially suitable for use in the production of highly polymeric poly (ethylene terephthalate) from the bis ethylene glycol ester of terephthalic acid since the colour of this polyester is of special importance in assessing its potential suitability for commercial applications. Examples of other aromatic dicarboxylic acids whose bis ethylene glycol esters may be converted to high polymer by the process of this invention include isophthalic acid, dicarboxdiphenyls, naphthalene dicarboxylic acids, and compounds containing two carboxyphenyl groups linked by a divalent bridging group which may be for example, alkylene, alkylene dioxy e.g. --OCH₂ CH₂ O--, --O--, --S--, --SO--₂ or --CO--. Mixtures of bis ethylene glycol esters of aromatic dicarboxylic acids may also be used.

As is well known, oxo compounds are compounds characterized by containing at least one oxygen atom which is bound to only one other atom and for the purposes of this specification an organic oxo compound is to be understood to mean a compound wherein the atom to which the oxygen is bound is in its pentavalent state, and wherein one of the three remaining valencies of the atom is satisfied by a carbon atom or an oxygen atom the other free valency of which is satisfied by a carbon atom, an oxygen atom the other free valency of which is satisfied by a carbon atom, or a further atom of phosphorus.

Examples of our specified organic oxo compounds are those having the structure ##STR1## where M is phosphorus each R is a monovalent hydrocarbyl group or a monovalent hydrocarbyloxy group preferably containing not more than 8 carbon atoms, each R₁ is a direct link or a divalent hydrocarbon group preferably containing not more than 2 carbons in the chain linking the M atoms and not more than 14 carbon atoms in all, and n is zero or a positive whole number which is generally one.

When n is zero in the structure I above, compounds having the structure II may be obtained. ##STR2## where M is phosphorus and each R is selected from alkyl, aralkyl, aryl, alkaryl, alkoxy, alkaryloxy, aryloxy and aralkyloxy where alk(yl) includes cycloalk(yl), especially methyl, ethyl, isomeric propyl, isomeric butyl, phenyl, methoxy, ethoxy, isomeric propoxy, isomeric butoxy and phenoxy.

Examples are trialkyl phosphine oxides, triaryl phosphine oxides, alkyl diaryl phosphine oxides, aryl dialkyl phosphine oxides, and trialkyl, triaryl, alkyl diaryl and aryl dialkyl -phosphates, -phosphonates and -phosphinates. (The term alkyl as used herein also includes cycloalkyl and aralkyl, and the term aryl includes alkaryl).

All the R's may be identical, if desired and representative examples are trimethyl phosphine oxide, triphenyl phosphine oxide, tri-n-butyl phosphine oxide, triphenyl phosphate, trimethyl phosphonate and triphenyl phosphinate. Alternatively, the R's may be different and examples are methyl diphenyl phosphine oxide, the methyl ester of methyl phenyl phosphinic acid, trimethyl phosphinate and triphenylphosphinate.

In the case where n is 1, compounds having the structure ##STR3## may be obtained where each R is selected from alkyl, aralkyl, aryl, alkaryl, alkoxy, alkaryloxy, aryloxy and aralkyloxy where alk(yl) includes cycloalk(yl); especially methyl, ethyl, isomeric propyl, isomeric butyl, phenyl, methoxy, ethoxy, isomeric propoxy, isomeric butoxy and phenoxy and R₁ is a direct link or a divalent hydrocarbon group preferably having not more than 2 carbon atoms in the chain linking the M atoms and not more than 14 carbon atoms in all. Compounds having this structure are the much preferred compounds for use in the catalyst compositions utilized in our invention. In these compounds, R₁ is preferably a direct link or a divalent hydrocarbon group, --CH₂ -- or --CH₂ CH₂ --, and M is phosphorus.

Representative of these compounds are: ##STR4##

While the R's are all identical in the compounds listed above, it is to be understood that those wherein the R's are different are also suitable for use in the process of our invention.

The antimony compound is preferably a trihalide e.g. antimony trichloride, antimony tribromide or, especially antimony trifluoride. However a tri(pseudohalide), e.g. tricyanide or triisothiocyanate, may also be used.

The amount of antimony compound used will generally be in the range 0.02% to 0.2% by weight of bis glycol ester although amounts as small as 0.008% or up to as great as 0.5% by weight may be used if desired. The organic oxo compound is preferably used in an amount which provides two oxo oxygen atoms for each antimony atom although larger or smaller amounts may be used, if desired. Thus, in the case of oxo compounds having the structure II above, it is preferred to use two moles/mole of antimony compound and in the case of oxo compounds having the structure III above, the preferred amount is one mole/mole antimony compound.

The two components of our specified catalyst system may be added separately or together at or before the commencement of the reaction. Alternatively, one or both of the components may be injected into the polycondensing system at a later stage, if desired, although some of the advantage of using a catalyst may then be lost. In yet a further alternative, the oxo compound may be added before or during any preliminary reaction to form the bis glycol ester but it is preferred to delay the addition of the antimony compound until this preliminary reaction is complete or nearly so.

The polycondensation, which may be effected batchwise or as a continuous reaction, is preferably effected at a temperature of at least 280° C in order to achieve desirable rates of reaction, and high vacuum is generally applied in order to aid the removal of glycol by-product. The reaction is usually continued until the desired molecular weight is achieved. The viscosity of the melt provides a measure of molecular weight and the progress of the reaction may be determined by observing the increase in melt viscosity with time.

One method of estimating the effectiveness of our process is with reference to the luminance of the polymeric products, increase in the value of which reflects a reduction in greyness. Luminance is a measure of the proportion of the incident light reflected on examination of the polymer using a "Colormaster" which is the trade name for the differential colorimeter manufactured by Manufacturers Engineering and Equipment Corporation. The luminance may be measured on the as made polymer or on articles, e.g. filaments, fibres, films or moulded articles, fabricated therefrom.

In the case of poly(ethylene terephthalates) in particular, relatively small variations in luminance can be of considerable importance in determining the value of a particular polymer for commercial purposes. It is thus highly desirable, particularly for filament end uses, that the polymer should have a high luminance value. However, luminance values depend to a considerable degree upon the history of formation of the polymer and comparisons of results found in the Examples are only meaningful where the polymers in question were prepared under identical conditions, on the same scale, and in the same equipment.

The invention is now illustrated by the following Examples in which all parts are expressed as parts by weight and all intrinsic viscosity measurements were recorded using solutions of 1 g. of polymer in 100 ml. of o-chlorophenol at 25° C.

EXAMPLE I

Experiment A.

100 Parts of dimethyl terephthalate and 71 parts of ethylene glycol were added cold to a stainless steel vessel which had previously been purged with nitrogen and which was provided with heating means, a stirrer, an offtake for volatile material and a nitrogen inlet and outlet. The mixture was then melted at 120°-130° C under nitrogen and an amount of manganese acetate tetrahydrate equivalent to 0.025 part of anhydrous manganese acetate was added. Gentle heating was then applied and the temperature was gradually raised to 220° C, and the methanol distilled off. When the theoretical amount of methanol had been collected, 0.015 part of phosphorus acid was added and the mixture was transferred under oxygen-free conditions to a stirred polymerisation autoclave. 0.074 part of antimony trifluoride and 0.5 part of titanium dioxide were then added and the temperature raised to 235° C. Pressure was then reduced to 1 mm. of mercury absolute while the temperature was raised to 290° C. The temperature was then maintained at 290° C until an adequate melt viscosity had been achieved, and the polymer then extruded and granulated. The polymeric product, intrinsic viscosity 0.71, had a luminance value of 54.

Experiment B.

The process was repeated but using only 0.037 part of antimony trifluoride (i.e. about 0.028% by weight based on bis glycol ester assuming 100% conversion of the dimethyl terephthalate) and, in place of the phosphorus acid, 0.063 part of tetraisopropyl methylene diphosphonate. The polymeric product, intrinsic viscosity 0.68, had a luminance value of 75. (The diphosphonate was prepared by the method described in the Journal of the American Chemical Society (1961) Volume 83 at page 1722).

EXAMPLE 2

The process of Experiment 1B was repeated but using only 0.019 part of antimony trifluoride and, in place of the diphosphonate, 0.043 part of tetraphenyl methylene diphosphine dioxide. The polymeric product, intrinsic viscosity 0.71, had a luminance value of 67.

(The tetraphenyl methylene diphosphine dioxide was prepared as follows: tetraphenyl methylene diphosphine (38.4 parts) in acetone (400 parts) was treated with 100 volume hydrogen peroxide (23 parts) in acetone (100 parts) at 0° C. The diphosphine dioxide so produced crystallised on evaporation of the solvent to about one third of the original volume of solution. It was recrystallised from hot acetone and dried in a vacuum oven at 120° C for 3 hours to yield crystals having a melting point of 185°-186° C, Yield: 75-85% 

What we claim is:
 1. A polyethylene terephthalate composition comprising highly polymeric polyethylene terephthalate and in an amount sufficient to prevent color degradation of said polyethylene terephthalate (1) a trihalide or tri(pseudohalide) of antimony, and (2) an organic oxo compound of phosphorus.
 2. The composition of claim 1, wherein said phosphorus compound is selected from the group consisting of tetraisopropyl methylene diphosphonate and tetraphenyl methylene diphosphine dioxide.
 3. The composition of claim 1, wherein the molar ratio of organic oxo compound to antimony compound is 1:1 up to 2:1 and the antimony compound is present in an amount of 0.008 up to 0.5% by weight of polyethylene terephthalate.
 4. A polyethylene terephthalate composition comprising highly polymeric polyethylene terephthalate and in an amount sufficient to prevent color degradation of said polyethylene terephthalate (1) an antimony trihalide and (2) a phosphorus compound having the formula ##STR5## where each R is selected from the group consisting of a monovalent hydrocarbyl group and a monovalent hydrocarbyloxy group. 